I don't have a lot to say about the basic argument, other than you should be deeply skeptical of anyone who tries to prop up obviously ideological contentions as empirical straw men in order to discredit the political opposition. Mann and Ornstein seem to avoid specifics for the most part, though here's how Froomkin summarizes what they're saying about the GOP:

Lies from Republicans generally and standardbearer Mitt Romney in particular weren't limited to the occasional TV ads, either; the party's most central campaign principles -- that federal spending doesn't create jobs, that reducing taxes on the rich could create jobs and lower the deficit -- willfully disregarded the truth.

These are awfully reductive and debatable statements—if this is how the GOP is willfully disregarding the truth, than the current Democratic insistence that a slight increase in marginal tax rates for the rich is a serious solution to address our exploding debt is problematic by the same standards.

But what I really find astounding is that Ornstein and Mann decided to weigh in on media fact checking. Mann and Ornstein's reputations rest on their vocation as policy experts, and their conclusions expose them as both ignorant and ideological. I actually agree with their conclusion "the fact-checkers may have made things worse rather than better," and that it's no substitute for thorough reporting. But they quickly run off the rails:

And then there was this terrible irony: "Fact checkers almost seemed obliged to show some balance in their fact checking."

"There was some damn good stuff done, and stuff that really did hold Romney to account," Ornstein said. But no fact-checker intent on "appearing to be utterly straightforward, independent, and without an axe to grind, is going to actually do the job of saying that we're going to cover 20 fact checks on one side, to three on the other."

So, Ornstein concluded: "If you looked at where the scales should have been, and where they were, they were weighted. And they weren't weighted for ideological bias. They were weighted to avoid being charged with ideological bias."

Here's the problem: we actually can quantify fact checking pretty easily. And contra Ornstein, the results are that fact checkers are pretty willing to say Republicans are the problem, credibility be damned. I've noted this many times before, but the University of Minnesota Humphrey School of Public Affairs did a study on PolitiFact, the largest media fact checking organization, from January 2010 through January 2011. The study found "current and former Republican officeholders have been assigned substantially harsher grades by the news organization than their Democratic counterparts. In total, 74 of the 98 statements by political figures judged 'false' or 'pants on fire' over the last 13 months were given to Republicans, or 76 percent, compared to just 22 statements for Democrats (22 percent)."

More recently, the Center for Media and Public Affairs at George Mason University tallied up 98 PolitiFact findings from June 1 to September 11: